The BGA is a surface mounting package developed for mounting a multipin circuit such as LSI on printed circuit board. In the BGA, solder material is formed into small balls, which are disposed at specific intervals in lattice on principal surface or other surface of IC package. They are soldered and mounted on the surface of soldering land provided on the printed circuit board.
FIG. 15 shows an example of printed circuit board for mounting the BGA. On a principal surface of the printed circuit board 100, plural soldering lands 101, 101a are provided, and wiring patterns 102 are connected to the soldering lands 101.
It is relatively easy to draw out a wiring pattern 102 from a soldering land 101 provided near the outer circumference of a printed circuit board 100 toward the outside of the printed circuit board 100. It is, however, difficult to draw out the wiring pattern connected to the soldering land 101a existing inside of the printed circuit board 100 toward the outside of the printed circuit board 100.
In the soldering land 101a which is hard to draw out the wiring from principal surface (or other principal surface) of printed circuit board 100, the wiring must be drawn out from other principal surface 100b (see FIG. 16) of printed circuit board 100 by opening a through-hole penetrating from one principal surface to other principal surface of printed circuit board 100.
FIG. 16 is a sectional view of connection parts when soldering the printed circuit board 100 having through-holes and BGA 103.
In FIG. 16, a plurality of terminals 104 are provided on principal surface 103a of BGA 103, and solder balls 105 are connected to terminals 104. No element is formed on other principal surface 103b of BGA 103. On principal surface 100a of printed circuit board 100, soldering lands 101 are provided corresponding to terminals 104 of principal surface 103a of BGA 103. Solder balls 105 are connected to soldering lands 101. Soldering lands 101 are drawn out to outside by way of wiring pattern 102. Conductors 109 are provided in the inner wall of through-hole upper lands 107 and through-holes 106, and are drawn out to the side of other principal surface 100b of printed circuit board 100 by way of through-hole lower lands 108.
Thus, when drawing out the wiring pattern 102 to the side of other principal surface 100b of printed circuit board 100, it is general to open through-holes 1006 penetrating through the printed circuit board 100. If pitch A becomes narrow between terminal 104 and adjacent terminal 104 of BGA 103, sufficient insulation distance (interval) cannot be provided between the through-hole upper lands 107 and soldering lands 101 (between B and C in FIG. 16), electric insulation reliability is lowered.
FIG. 17A shows a case of setting pitch P104a of terminals 104 of BGA 103 at 1.0 mm. At this time, diameter φ110 of soldering lands 110 is 0.5 mm, and diameter φ106 of through-holes 106 is 0.3 mm. Diameter φ107 of through-hole upper lands 107 is 0.6 mm.
In the configuration in FIG. 17A, interval L17a of soldering land 110 and through-hole upper land 107 is 157 μm. This interval is sufficient in relation to the necessary insulation distance of 100 μm for preventing electric short-circuit accident between through-hole upper lands 107 and soldering lands 110, and a sufficient electric insulation is assured.
FIG. 17B shows a case of setting pitch P104b of terminals 104 of BGA 103 at 0.8 mm. At this time, diameter φ110 of soldering lands 110 is 0.4 mm, and diameter φ106 of through-holes 106 is 0.3 mm. Diameter φ107 of through-hole upper lands 107 is 0.6 mm.
In the configuration in FIG. 17B, interval L17b of soldering land 111 and through-hole upper land 107 is 66 μm. This interval is not satisfactory in relation to the necessary insulation distance of 100 μm for preventing electric short-circuit accident between through-hole upper lands 107 and soldering lands 110. Therefore, unlike FIG. 17A, it is not sufficient to satisfy electric insulation.
Degree of pitch of BGA (pitch A in FIG. 16) varies with the increasing functions of digital appliances such as cellphones and portable digital music players, and downsizing tendency. Narrow pitches are preferred recently, and a terminal width of less than 0.5 mm is known. It is estimated to be difficult to develop inexpensive printed circuit boards applicable to such narrow pitches.
Various methods have been proposed to assure sufficient electric insulation and prevent electrical short-circuit between through-hole upper lands 107 and soldering lands 101 placed at positions of different potential.
For example, it is proposed to decrease the diameter φ106 of through-holes 106 and diameter φ107 of through-hole upper lands 107. This is shown in FIG. 18. In FIG. 18, diameter φ112 of soldering lands 112 is 0.4 mm, diameter φ106 of through-holes 106 is 0.25 mm, and diameter φ107 of through-hole upper lands 107 is 0.5 mm. If the through-holes 106 are in this size, a sufficient distance can be assured between the through-hole upper lands 107 and soldering lands 112.
However, to narrow diameter φ106 of through-holes 106, a fine drill must be used when forming through-holes in printed circuit board 100. Accordingly, the thickness of printed circuit board must be reduced to a specified thickness. This method is hence difficult if the thickness of printed circuit board cannot be reduced as desired.
Diameter φ107 of through-hole upper lands 107 can be reduced without changing diameter φ106 of through-holes 106. In this case, through-holes 106 must be disposed at specified positions at high precision, and the manufacturing yield and processing yield of printed circuit board are lowered, and the cost of printed circuit board is increased.
To assure insulation distance between soldering lands of different potentials within a specified interval, it may be considered to minimize diameter φ112 of soldering lands 112. However, if diameter φ112 of soldering lands 112 is reduced, cracks are likely to be formed, and mechanical and electrical connection reliability is lowered.
Moreover, to assure insulation distance between soldering lands of different potentials within a specified interval, a method called pad-on-via is known. As shown in FIG. 19 and FIG. 20, vacant portion of through-hole 106 is filled up, and soldering land 113 is provided on through-hole 106. As a result, the through-hole 106 and soldering land 113 are disposed at the same position, and insulation distance from other soldering lands can be assured.
However, IVH (interstitial via hole) substrate or build-up substrate shown in FIG. 19 or FIG. 20 is expensive, and the substrate 100 filling up the through-holes 106 is also expensive as compared with general substrate having through-holes of penetrate-through type. In FIG. 19 and FIG. 20, through-holes 106 and soldering lands 113 are also shown.
In addition, to assure insulation distance between soldering lands of different potentials within a specified interval, another method is known as shown in FIG. 21, in which through-hole upper lands 107 are used as soldering lands (see, for example, Japanese Patent Application Laid-Open No. 2001-168511).
In this method, however, the amount of solder loaded into each through-hole 106 is not constant. As a result, the amount of solder for connecting the terminals 104 and through-hole upper lands 107 varies, and satisfactory reliability cannot be maintained in mechanical and electrical connection. In FIG. 21, BGA 103 and solder balls 105 are also shown.